Determining the position and direction of a conductive pipe (metallic casing, for example) accurately and efficiently is required in a variety of downhole applications. Perhaps the most important of these applications is the case of a blown out well in which the target well must be intersected very precisely by a relief well in order to stop the blowout. Other important applications include drilling of a well parallel to an existing well in Steam Assisted Gravity Drainage (“SAGD”) systems, avoiding collisions with other wells in a crowded oil field where wells are drilled in close proximity to each other and tracking an underground drilling path using a current injected metallic pipe over the ground as a reference.
A number of conventional approaches have attempted to provide solutions to this problem. In one method, current is induced on a target casing by transmitting electromagnetic waves via coil antennas. This induced current in turn causes the casing to radiate a secondary electromagnetic field. The amplitude of this secondary field can be used to determine the distance to the target casing. However, since the amplitude of the field is strongly dependent on the properties of the casing and the formation, the accuracy of this method can remain low.
In another conventional approach, an electrode type source is used to induce current on the target casing to thereby generate a magnetic field. Gradient of the magnetic field radiated by the target casing, in addition to the magnetic field itself, is measured in this approach. By using a relationship between the magnetic field and its gradient, an accurate ranging measurement is made. However, since electrodes are sensitive to the resistive oil-based muds, the electrode must be positioned in direct contact with the formation to inject the current. As a result, high contact losses may occur, or ohmic losses in highly resistive formations may reduce the range of the tool.
Accordingly, there is a need in the art for improved downhole ranging techniques.